Electrochromic materials undergo a change in optical properties when ions and electrons are inserted into them under the influence of an electric field. An electrochromic device may be constructed from such materials, such that the visual transmittance of said device changes when an electrical potential is applied between two electrodes. Electrochromic devices have many applications, including switchable glazings (where transmission of heat and light is regulated according to some scheme), automotive mirrors (where mirrors switch to prevent the driver from being dazzled by glare, etc.) and in display materials (where electrochromic devices take the folio of display elements to form switchable images).
The application of electrochromic materials in window glazings requires that the electrodes are of large area (>1 m2) and operate at temperatures ranging from −20° C. to +80° C. These constraints place specific requirements on the materials available for electrochromic device construction, and affect the switching characteristics. The successful commercialisation of electrochromic devices as window glazings requires a substantial switching lifetime (>10 years), sufficient optical transmission homogeneity and reasonable switching time (<15 minutes); all of these factors are dependent on the switching algorithm used. The application of excessive electrical potentials causes side reactions to occur which result in device degradation and reduction of useful lifetime. The application of inappropriately low potentials results in excessive switching times. Switching of electrochromic devices must then be carried out in a manner which satisfies the requirements outlined above.
The useful lifetime of electrochromic devices depends primarily on the magnitude of the applied electrical potentials and on the amount of charge inserted into the electrochromic layers; the limits for these parameters may be readily determined by electrochemical experimentation. If the reversible charge injection limit for the electrochromic layers is determined, and layers are not overcharged during device switching (i.e. the reversible limits are not exceeded), it is then the applied potential which has the greatest influence on device lifetime.
The range of potentials which may be applied between the electrode layers, without causing device degradation is often referred to as the redox stability range; the application of potentials outside this range causes device degradation thereby reducing lifetime. The redox stability range may be determined, for example, by cyclic voltammetry experiments at various temperatures. The optimisation of useful lifetime may then be made by simply limiting the electrical potential between the electrode layers to the redox stability range for that particular system. The difficulty in applying this idea lies in the fact that the potential is generally applied between two electrical contacts, which are on opposite sides of the electrochromic device (as shown in FIG. 1).
Typical electrochromic device structures as known from the state of the art comprise substrates (usually glass), electrode layers (electrically conducting), electrochromic layers and electrolyte (polymer or inorganic). Electrical contacts to the electrode layers are provided.
When a potential is applied between the contacts (contact potential, UAB), a distributed electric field is generated between the electrode layers. The resistivity of the electrode layers is relatively high compared to metallic conductors (ca. 10-20 Ohm/sq) which results in a significant potential drop across each of the electrode layers. The resulting potential difference between the electrode layers at a given point x (Uf(x)), is then a function of displacement of point x from the electrode contacts. If an electrochromic cell has only two contacts it is not possible to directly measure the potential between the electrode layers Uf(x). In order to ensure that the potential between electrode layers is within the redox stability range, it is necessary to estimate Uf(x) or measure it directly (in which case at least three contacts are required).
The potential distribution described above is such that Uf(x) is highest adjacent to cell contacts, and is lowest midway between the cell contacts. This causes switching (colouring and bleaching) to occur faster at the edges of the cell (near the contacts) than in the middle of the device (between the contacts); the so-called ‘edge effect’. As the potential between the electrode layers is highest at the contacted edges of the cell, it is not necessary to simulate the potential distribution over the entire cell; it is sufficient to correlate the potential applied to the cell contacts with the maximum potential generated between the electrode layers (Uf,max). The applied potential may then be limited accordingly, thereby ensuring that the maximum potential generated between electrode layers Uf,max remains within the safe redox limits.
Switching with high currents allows for faster response and therefore lower switching times, however results in higher inhomogeneity of transmission. The distribution of electrical potential between the electrode layers of a cell depends inherently on the resistance of the electrode layers and the cell current. High currents cause a greater internal potential drop across the electrode layers, which results in a less homogeneous potential distribution. In order to switch electrochromic devices with more homogeneous optical transmission, it is then useful to limit the cell current, however switching time becomes unacceptably long if current is too low. Fast switching and homogeneous switching are then mutually competitive aims, and a balance must be found between the two in order to optimise switching characteristics. It is then inherently necessary to be able to control cell current in order to switch cells with reasonable speeds and transmission homogeneity.
WO9837453 describes a method for switching electrochromic devices, according to which the preamble of the patent claim is formulated. This method involves switching an electrochromic device by applying a potential ramp from zero volts up to a predetermined temperature-dependent limit ‘Umax’. The current is continuously measured during the potential ramp, and the total resistance of the cell ‘Rges’ is calculated from the potential and current data. The effective potential at the electrochromic layers ‘Ueff’ is calculated from the applied potential, the cell current and the total resistance (‘Rges’). The potential is applied such that ‘Umax’ and ‘Ueff’ are limited to temperature-dependent predetermined limits, until the desired optical/charge state is obtained.
This method has the following disadvantages:                1. The application of the electrical potential according to this method relies on the total cell resistance (‘Rges’), which is described as the sum of all of the ohmic resistances, between the cell contacts. The method theorises that this resistance may be used to ensure that safe electrochemical potential limits are not exceeded, during switching. The total cell resistance includes series resistances from cables, electrode layers, electrochromic layers and electrolyte. However, it has been experimentally shown that this theory does not work in practise, and the use of the resistance ‘Rges’ in controlling the switching of electrochromic devices results in the application of electrical potentials significantly exceeding safe electrochemical limits.        2. The method involves the calculation of the effective potential at the electrochromic layers ‘Ueff’, which does not correspond to a discrete physical quantity. It is assumed that limiting the applied electrical potential according to ‘Ueff’, will prevent degradation from occurring. In actual fact, limiting the applied electrical potential according to ‘Ueff’, results in electrical potentials between the electrode layers which significantly exceed safe redox stability limits.        3. The method does not allow for optimisation of transmission homogeneity or switching speed as current is not controlled.        4. The method only allows for switching between completely coloured and bleached states.        
EP 0 475 847 B1 describes a method for switching an electrochromic device by first applying a small voltage pulse, during which time the current is measured and used to estimate temperature and hence select an appropriate final switching potential. The switching is carried out under constant potential until either the current density reaches some predetermined threshold limit, the charge density reaches 10 mC/cm2 or the desired transmission level is reached. This method also provides provisions for switching of an electrochromic device with three electrodes, whereby a potential difference is applied to two electrode contacts (contacts 1 and 2), such that the potential difference between contacts 2 and 3 remains constant.
This method does not allow for optimisation of transmission homogeneity or switching speed as current is not controlled. The use of electrochromic devices with three contacts allows the potential between the electrochromic layers to be accurately and safely controlled, provided that safe redox limits are not exceeded. Such devices are disadvantageous however, because the production process is unduly complicated as substrates must be cut to size and masked before coating (to allow for the third contact), thereby increasing production time and financial cost.
EP 0 718 667 A1 describes a system for switching electrochromic devices with two contacts, using a controller unit with user-interface, a power generator, temperature sensor, etc. This patent details methods for switching using either constant potential or constant current, with specific conditions and safety criteria provided for each method. The constant potential method involves application of a predetermined temperature-dependent potential to the cell until either the required charge density is reached or the current drops below some threshold value. The constant current method involves application of the current until either the required charge density is reached or the current drops below some threshold value. The second differential of potential with respect to time is calculated continuously, and the current set point is reduced (halved) if this exceeds some predetermined limit.
This method has the disadvantage that it limits either cell current or voltage, and not both. The constant current operation described in this method may be used to optimise switching speed and transmission homogeneity, however does not ensure that the potentials between electrode layers are within the safe redox limits. The potential generated between the electrode layers may be monitored using a 3-electrode system, however this increases complexity and expense. The reduction in applied current by 50% based on the second differential of potential with respect to time is a novel solution, however does not guarantee that safe redox limits will not be exceeded. The constant potential method does not allow for control of the cell current, hence cannot allow for optimisation of switching speed and transmission homogeneity simultaneously.
U.S. Pat. No. 7,277,215 B2 describes a system for switching electrochromic devices with three electrode contacts. Two of the electrodes are used for cell switching and the other two are used for heating and/or breakage detection. The microcontroller uses a variety of AC/DC techniques to colour and bleach the device, measure temperature, heat the device and detect mechanical breakage of the pane. The controller uses an internal circuit which is matched to external conditions in order to calculate leakage current and adjust switching parameters accordingly. This system is extremely complicated and requires that the internal circuitry of the controller be well matched to the specific properties of the EC device. The disclosed system offers a novel solution to the problems concerned with leakage current in EC devices with inorganic ion conductors, however is unduly complex for systems with organic ionic conductors.
Despite these advances, there remains a need for a switching method for electrochromic devices which:                1. ensures that the potential between the electrode layers is always between safe redox limits, even for devices with two electrode contacts;        2. allows for optimisation of switching speed and transmission homogeneity, via limitation of the applied cell current.        